Fluid distribution systems are well known in the art. One example of a fluid distribution system is the system associated with heating, ventilating and air-conditioning (HVAC) distribution systems. HVAC distribution systems see widespread use in commercial applications, i.e., residential housing, apartment buildings, office buildings, etc. However, HVAC distribution systems also see widespread use in laboratory-type settings. In this implementation, the HVAC distribution system is primarily intended for clean-room use, exhaust of potentially noxious fumes, etc.
In a majority of HVAC distribution system implementations, the primary goal is to produce and distribute thermal energy in order to offset the cooling and heating needs of a particular installation. For purposes of analysis, the distribution system can be divided into two subsystems; global and local subsystems. The global subsystem consists of a prime mover (i.e., a source) which might be a fan in an air distribution system or a pump in a water distribution system. Also included in the global subsystem is the duct-work required to connect the global subsystem to the local subsystem. The local subsystem primarily consists of dampers, valves or fume hoods.
Current control practice, in both commercial and laboratory HVAC distribution systems, separates the global subsystem from the local subsystem and accordingly treats the individual subsystems independent of one another. The result of this separation is (1) poor controllability, (2) energy waste throughout the system, and (3) costly commissioning (installation and maintenance) processing.
The current state-of-the art control strategy for heating and cooling needs is based on varying the volume of air flow as the space thermal load changes. Thus, variable air volume (VAV) systems have become very popular for HVAC distribution systems. The underlying principle of the VAV system is to reduce the cost as the thermal load diminishes. Both the cost of moving the air, i.e. fan energy, and the thermal energy cost to condition the air will reduce with the decrease in volume of air. Since the modulation of the rate of flow of air is necessary, the air distribution fan is equipped with some means of varying the volume of air the fan delivers. This is typically done by varying the speed of the fan.
FIG. 1 generally depicts a prior art HVAC distribution system. As depicted in FIG. 1, a fan controller 103 controls the variable air volume by controlling the speed of a fan 106 so that a constant static pressure at an arbitrary duct location (for example, the location 114) is maintained. A damper 118 is controlled by a damper controller 124. The static pressure at the location 114 fluctuates as the flow requirement of the damper 118 varies. However, the fan controller 103 ignores the requirement of static pressure in the entire system so that the flow requirement of the damper 118 can be satisfied. In this scenario, the fan controller 103 attempts to maintain an arbitrarily selected pressure setpoint, which is often set based on a maximum operating design condition. During normal operating conditions, however, the system static pressure requirement is considerably lower than the design condition. This results in a considerable amount of energy waste since the fan continuously operates to satisfy the maximum static pressure setpoint. If, on the other hand, the setpoint is much lower than the system requirement, the system is incapable of satisfying the flow requirements, which results in an ineffective system. In addition, no scientific methods exist to determine the best (optimum) position of the static pressure sensor 112 within the duct 115. In other words, the positioning of the static pressure sensor 112 is more of an art than a science. Furthermore, the tuning of the VAV fan control can be time consuming and costly if the selected pressure setpoint and the position of the static pressure sensor 112 are chosen incorrectly.
Another problem associated with the implementation of FIG. 1 is the fixed setpoint for static pressure sensor 112. As VAV boxes 109, 118 are opened and closed, the pressure throughout the system will decrease/increase accordingly. By fixing the setpoint while the pressure fluctuates, energy is wasted because the static pressure is not reduced when VAV boxes 109, 118 do not demand much flow.
For the sake of simplicity, if an air system can be assumed to be a closed loop, then the distribution system can be modelled as a parallel electrical circuit. Each branch of the circuit can be represented by a VAV box supplying air to a zone and associated branch ducts/fittings. Typically, in a VAV system the static pressure sensor (like sensor 112) is located in the farthest (or 2/3 downstream in the air duct) branch. Placement at this location assumes the pressure in this branch will be the maximum considering the length of ducts connecting this branch and the fan.
In reality, however, the maximum branch pressure loss may be dictated by another branch. For example, if a branch closer to the fan has high pressure loss components, the pressure of the fan P.sub.f, will be less than what is required. As a consequence, the branch near the fan will not receive the required air flows.
One technique of VAV fan control is based on measuring branch flows and modulating the fan speed to achieve the desired flow Q.sub.f exiting the fan. This technique is described by Warren, M., et al., "Integrating VAV Zone Requirements with Supply Fan Operation," ASHME Journal, April 1993 and Haman, T. B., "TRAV--A New Concept," Heating/Piping/Air Conditioning, July 1989. The shortcoming of this approach is that the fan energy output is based on two variables, static pressure and flow. The use of a measured flow as a process variable requires a separate feedback loop. In this case, the fan controller will search for the correct static pressure such that all the branch flow requirements are satisfied. However, the feedback loop may have severe limitations in measuring and exchanging the flow information over the network. Thus, while the response is slow, controllability is likewise affected since the fan controller will continually search for the correct static pressure to generate.
Another technique of VAV fan control is to measure the damper position in each branch, and modulate the fan speed until one of the dampers remains almost 100% open. This technique is described by Hamnan, T. B., "Direct Digital Controls for HVAC Systems," McGraw-Hill Inc., 1993. This technique assumes that when at least one damper is nearly 100% open, the minimum limit of the fan static pressure is achieved. This strategy reduces fan energy cost, but may not reach the optimal solution since damper position cannot sufficiently determine the fan control point. Furthermore, the communication problem of data flow and additional cost of positioning sensors remain an issue.
Okada, T., et al., "Research and Development of Home Use VAV Air-Conditioning System," ASHRAE Transactions, V.98, Pt. 2, 1992 describe a method of calculating branch resistance for a residential HVAC distribution system by opening one damper at a time. This method, however, is impractical in non-residential buildings due to the presence of large numbers of VAV boxes in such applications.
Goswami, D., "VAV Fan Static Pressure Control with DDC," Heating/Piping/Air Conditioning, December 1986 describes a method of on-line determination of fan static pressure. This method assumes that control software is able to calculate the system pressure loss in real-time for a given system flow requirement. The method is costly, however, since the control software must have detailed duct information, system layout and pressure loss data to compute the fan static pressure requirement. Additionally, as the distribution system undergoes changes, the data base must be updated making implementation of this approach even more costly. Finally, it may be impossible to develop such data in certain retrofit situations due to non-accessibility to the duct system.
Thus, a need exists for a control system which, when implemented in a HVAC distribution system, efficiently controls a prime mover without the energy waste and cost inherent in the prior art.